This invention relates generally to rotary spray grainers and more particularly to the use of a deflector plate in a spray grainer to provide distinct spraying and drying zones for the formation of spray grained calcium hypochlorite.
The use of a rotary drum apparatus to apply coatings or encapsulations to particulate substrates by the spraying of slurries, melts, or solutions and suspensions onto moving beds of particles that fall in cascades within the apparatus is well known. The application of distinct and separate coatings to a particulate substrate is known as granulation and is widely used in the fertilizer industry, for example in the production of ammonium nitrate. The application of coatings or encapsulations on a particulate substrate can also be employed in the making of confections, such as sugars, or, as in the case of the instant invention, in the manufacture of calcium hypochlorite for use as a commercial bleaching and sanitizing agent, particularly in the disinfection of swimming pool waters.
Previous attempts to employ a rotary drum dryer in the process of manufacturing spray grained particles have employed either a deflector means that runs the entire length of the rotary drum and which overlies the spraying means or nozzles or, in the alternative, utilize no deflector means in the rotary drum. Both of these approaches have their disadvantages which prevent the rotary dryer or spray grainer from achieving its optimum operating efficiency.
In apparatus which does not employ a deflector plate and which is rotated at a sufficient speed to obtain a full cascade of falling particles across the entire cross-sectional area, the slurried material being sprayed on the particulate particles does not achieve optimum spray pattern dispersion to uniformly wet the particles. The slurry is formed from a solid or semi-solid that is reduced to a sprayable liquid state by melting or by dissolution or suspension in an appropriate solvent. Frequently in this type of apparatus, the slurry will penetrate the falling wall or cascade of particulate substrate or particles and buildup on the interior walls of the drum, creating scaling. Alternately, in this type of apparatus the falling cascade of the particulate substrate will fall in a path that causes it to strike the spraying nozzles or spraying means, thereby causing the spraying means or nozzles to clog and buildup an accumulation of the slurry being sprayed.
Another approach not utilizing a deflector plate or deflector means in a rotary spray grainer uses rods disposed substantially parallel to each other and to the longitudinal axis of the drum radially inwardly of the interior walls of the drum. These rods facilitate the falling of particles in cascades or curtains so that the agglomerating agent or slurry may be sprayed into the cascades. However, this design still suffers from the disadvantage of permitting the particles to impinge on the spraying nozzles and thereby cause clogging.
In rotary drum dryers not utilizing deflector plates or other deflector means, avoidance of the impingement of particles on the spraying means has been achieved by keeping the drum rotational speed sufficiently low. This permits the nozzles or spraying means to disperse the slurried material to achieve the optimum spray pattern dispersion to uniformly wet the falling particles. While this avoids the impingement problem, it adversely affects the quality of the spray grained product obtained and reduces the capacity of the dryer. Both of these results are clearly not desireable.
In other rotary drum dryer apparatus using the spray graining principle, deflector means or deflector plates were provided that ran substantially the entire length of the drum. This full length deflector plate would protect the spraying means or nozzles from impingement by the falling cascade of particulate substrate and would avoid the concomitant clogging of the nozzles due to slurry buildup. The deflector plate also provides a dense cascade of falling particles at a fixed distance from the spray nozzles to permit a full slurry spray pattern to be obtained. However, the full length deflector means causes channeling of the hot air or gas used to dry the sprayed particles and prevents a full cascade from being obtained across the entire width or cross-sectional area of the drying zone during operation. This results in a lower heat transfer factor that gives poor quality spray grains and requires a longer drying time because of the less efficient use of the available heat energy.
These problems are solved in the design of the present invention by employing deflector means in a rotary drying drum overlying the spraying nozzles in the spraying zone, but not extending into the drying zone.